Hemoglobins show a very great range in their functional properties and hemoglobin systems often show striking adaptations to environmental pressures. In the freshwater trout, and in a number of marine fish, functionally diverse hemoglobin components are present. Some hemoglobin components, exemplified by Trout Hb IV, show large pH and cofactor effects in oxygen binding. The properties of these components may permit the fish to control the gas content of the swim bladder and thereby its hydrostatic equilibrium. The exaggerated effects of pH and anions on the oxygen binding properties of these proteins makes them useful as probes of allosteric control mechanisms. Other hemoglobin components, exemplified by Trout Hb I, show no pH dependence in oxygen binding and are essentially unaffected by variations in temperature or organic phosphate concentration. These may serve as physiological "back-ups" that allow for oxygen uptake and delivery in spite of environmental or physiological perturbations. Exploration of structure-function relationships in these unusual hemoglobins is of considerable interest since cooperative interactions are divorced from pH and anion effects. In an effort to understand the similarities and differences between the hemoglobins of trout and man, we have undertaken analysis of the primary structure of the major trout hemoglobins. These studies complement work in progress on the functional properties and the three-dimensional structures of the trout hemoglobins. In functional studies of various fish hemoglobin systems we stress the importance of characterizing isolated components. This should permit resolution of significant "alpha" and "beta" chain differences and conformation transitions that would be obscured in heterogeneous samples. Interpretation of the functional consequences of enzymatic modification is also greatly facilitated by use of purified hemoglobin.